The Cretaceous–Paleogene (K–Pg) transition saw mass extinctions in terrestrial and marine ecosystems. Terrestrial vertebrate diversity patterns across the K–Pg boundary have seen extensive study, but less is known about marine vertebrates. We describe a new mosasaurid from the latest Maastrichtian phosphatic beds of Morocco, showing how mosasaurids evolved to become apex predators in the latest Cretaceous. Thalassotitan atrox gen. et sp. nov., from the Oulad Abdoun Basin of Khouribga Province, Morocco is characterized by large size, a broad skull, massive jaws, and reduced cranial kinesis, suggesting it was highly adapted for carnivory. Teeth resemble those of killer whales in their robust, conical shape, and show heavy wear and damage. Phylogenetic analysis recovers Thalassotitan as a close relative of Prognathodon currii and P. saturator within the Prognathodontini. Among the associated fauna, three genera of mosasaurids, elasmosaurid plesiosaur, chelonioid turtle, and enchodontid fish show acid damage, and could be prey ingested by mosasaurids, likely Thalassotitan. Thalassotitan shows mosasaurids evolved to fill the marine apex predator niche, a niche occupied by orcas and white sharks today. Mosasaurs continued to diversify and fill new niches until their extinction at the end of the Cretaceous.

白垩纪-古近纪 (K-Pg) 过渡期见证了陆地和海洋生态系统的大规模灭绝。跨越 K-Pg 边界的陆地脊椎动物多样性模式已经得到广泛研究，但对海洋脊椎动物知之甚少。我们描述了摩洛哥最新的马斯特里赫特磷酸盐层中的一种新的沧龙科动物，展示了沧龙科动物如何在最新的白垩纪进化为顶级掠食者。Thalassotitan atrox gen. 等人。11 月 1 日，来自摩洛哥 Khouribga 省的 Oulad Abdoun 盆地，其特点是体型大、头骨宽阔、下颚大、颅骨运动减少，这表明它非常适合食肉动物。牙齿与虎鲸的牙齿相似，具有坚固的圆锥形形状，并显示出严重的磨损和损坏。系统发育分析恢复了 Thalassotitan作为Prognathodontini 内的Prognathodon currii和P. saturator 的近亲。在相关的动物群中，沧龙科、鱼尾龙、龟类和内齿鱼类三个属显示出酸损伤，并且可能被沧龙科的猎物摄取，可能是Thalassotitan。Thalassotitan表明，沧龙科动物进化到填补海洋顶级捕食者生态位，今天逆戟鲸和白鲨占据了这个生态位。沧龙继续多样化并填补新的生态位，直到它们在白垩纪末灭绝。